In-situ combustion processes are applied for the purpose of heating heavy oil, to mobilize it and drive it to an open production well for recovery.
In general, the usual technique used involves providing spaced apart vertical injection and production wells completed in a reservoir. Typically, an injection well will be located within a pattern of surrounding production wells. Air is injected into the formation, the mixture of air and hydrocarbons is ignited, a combustion front is generated in the formation and this resulting combustion front is advanced outwardly toward the production wells. Or alternatively, a row of injection wells may feed air to a laterally extending combustion front which advances as a line drive toward a parallel row of production wells.
In both cases, the operator seeks to establish an upright combustion front which provides good vertical sweep and advances generally horizontally through the reservoir with good lateral sweep.
However, the processes are not easy to operate and are characterized by various difficulties.
One such difficulty arises from what is referred to as gravity segregation. The hot combustion gases tend to rise into the upper reaches of the reservoir. Being highly mobile, they tend to penetrate permeable streaks and rapidly advance preferentially through them. As a result, they fail to uniformly carry out, over the cross-section of the reservoir, the functions of heating and driving oil toward the production wells. The resulting process volumetric sweep efficiency is therefore often undesirably low. Typically the efficiencies are less than 30%.
It would therefore be desirable to modify the in-situ combustion technique so as to better control the way in which the combustion gases flow and the front advances, so as to increase the volumetric sweep efficiency. The work underlying the present invention was undertaken to reach this objective.
The invention, in its preferred form, incorporates aspects of two processes which are known in the art.
Firstly, it is known to initiate the combustion drive at the high end of a reservoir having dip and propagate the combustion front downstructure, isobath-wise. This procedure to some extent reduces the problem of gravity segregation of the combustion gases, because the gases are forced to displace the oil downward, in a gravity influenced, stable manner.
Secondly, Ostapovich et al, in U.S. Pat. No. 5,211,230, disclose completing a vertical air injection well relatively high in the reservoir and a horizontal production well relatively low in the reservoir. The production well is positioned transversely relative to the combustion front emanating from the injection well. The production well is spaced from the injection well. By implementing this arrangement, the combustion front follows a downward path, toward the low pressure sink provided by the production well and the benefit of gravity drainage of heated oil is obtained. These effects enhance the sweep efficiency of the process and facilitate the heated oil reaching the production well. However, the premature breakthrough of the combustion front at a locus along the length of the transverse, horizontal leg will result in leaving an unswept reservoir zone between the leg's toe and the breakthrough locus.
The present invention will now be described.